The present invention provides a waste water treatment method for biologically treating waste water by positioning a waste water inflow port 22 at a position lower than a position of a treated water outflow port 24 and by using a semi-batch biological treatment tank 10 that stores biological sludge in the tank. The waste water treatment method includes: a biological treatment step for biologically treating the waste water with the biological sludge by stirring the waste water in the semi-batch biological treatment tank 10 in a state where introduction of the waste water through the waste water inflow port 22, and discharge of treated water through the treated water outflow port 24 are stopped; and a waste water introduction and treated water discharge step for starting introduction of the waste water through the waste water inflow port 22 and discharge of the treated water through the treated water outflow port 24 in a time period from stoppage of the stirring of the waste water in the semi-batch biological treatment tank 10 until a sludge blanket of the biological sludge is formed in the semi-batch biological treatment tank 10, wherein the biological treatment step and the waste water introduction and treated water discharge step are sequentially and repeatedly performed.